tector (200-700 nm), and stainless-steel columns (250 x 5 mm) packed with Silasorb-600 silica gel (Lachema, CSSR) with an average particle size of 10 pm.
Tahta, Vol. 34, No. I, pp. 223-226, 1987 Printedin Great Britain. All rightsreserved
0039-9140/87$3.00+ 0.00 Copyright0 1987Pergamon Journals Ltd
LIQUID CHROMATOGRAPHY OF PALLADIUM AND NON-FERROUS METAL CHELATES WITH l-(2-PYRIDYLAZO)-2-NAPHTHOL Yu.
S. NIKITIN, N.
B. MOROZOVA, S. N. LANIN, T. A. B~L’SHOVA, E. M. BASOVA
V. M. IVANOV and
Department of Analytical Chemistry, M. V. Lomonosov Moscow State University, Lenin Hills, Moscow, USSR (Receioed 15 October 1985. Revised 14 February 1986. Accepted 9 August 1986) Smnmary-1-(2-Pyridylaxo)-2-naphthol has been used for extraction concentration of palladium, copper, cobalt and nickel from aqueous solutions and subsequent separation of the chelates obtained, by means of high-pressure liquid chromatography. A technique for determining microamounts of palladium in aqueous solutions in the presence of RN-fold ratio of copper and 80-fold ratio of cobalt has been developed.
The rapid development of high-pressure liquid chromatography (HPLC) in recent years has considerably expanded the area of its application in science, technology and industry. In addition to the traditional fields, such as separation and determination of or-
ganic compounds and biologically active substances, and in environmental pollution monitoring, HPLC is widely used for inorganic systems for the separation and determination of metals both in the ionic state and in the form of organometallic and complex compounds.‘*2 Chelates appear to be convenient spe ties for the determination of some metals by HPLC,’ since the structures and properties of chelates are close to those of organic compounds, which makes possible the direct transfer of the wide experience obtained in organic HPLC. A combination of preliminary extractive concentration of chelates followed by chromatographic analysis of the extracts makes it possible to combine in a single technique the main advantage of HPLC, namely, fast and effective separation of complex multi-component mixtures, with the selectivity of complexation and extraction. The high molar absorptivity of chelates in the ultraviolet and visible regions of the spectrum allows the use of the detector most widely used in liquid chromatography, the spectrophotometer. The main principles for selecting the chelating agents for use in HPLC are well known. The requirements are that each element should form only one complex with the reagent, and that the chelates should be stable in the conditions for the chromatographic separation (medium, pH, temperature), and have high molar absorptivity at the absorption maxima.’ All the possible types of interaction of chelate complexes with silica gels have been identified,3*’ and mechanisms proposed for the separation of diethyldithiocarbamates5 and b-diketonates’,’ of metals by
thin-layer chromatography. Owing to the similarity of the adsorption process in two-dimensional and three-dimensional layers of a sorbent, these mechanisms can be used for interpretation of the chelate retention in HPLC. At present HPLC is mainly being developed in terms of choosing the optimal chelating systems for metal separation.’ Detection of platinum metals in the presence of non-ferrous metal impurities represents one of the important and not yet completely solved problems of the analytical chemistry of these metals. Separation and determination of the platinum metals in the form of their chelates holds a great deal of potential for solution of the problem. Use has been made&‘* of the chromatographic behaviour of palladium chelates with quadridentate ligands (various substituted fl-ketoimines and salicylaldimine) to separate palladium from copper and nickel, but the palladium was not determined quantitatively. The most promising class of organic reagents for platinum metals, palladium in particular, since these tend to form complexes with donor S- and N-atoms, appears to be that of the heterocyclic axe-compounds which are widely used for their spectrophotometric determination.12 l-(2Pyridylazo)-2-naphthol (PAN-2), one of this class of compounds, complies with all the requirements listed above for chelating agents useful in HPLC. PAN-2 has already been used for the separation and determination of iron, nickel, cobalt and copper by both normal-phaseI and reversedphaseI HPLC. This paper deals with a study of the chromatographic properties of the palladium, copper, nickel and cobalt chelates with PAN-2 and the development of an HPLC method which would combine preliminary extractive concentration of palladium with its determination in the presence of larger amounts of copper, nickel and colbalt. 223
Yu. S. NIKITINet al.
224 EXPERIMENTAL
Palladium solution (0.935 mg/ml) was prepared by dissolving PdCl, (analytical grade) in 0. IM hydrochloric acid, and standardized gravimetrically with dimethylglyoxime.r5 Palladium solutions of lower concentration were prepared by dilution with O.OlM hydrochloric acid. Copper, cobalt and nickel solutions were made from the pure nitrates and standardized complexometrically. Inorganic salts were removed from the PAN-2 reagent (Reanal, Hungary) by recrystallization from ethanol,16 and the absence of PAN-l in the initial PAN-2 was checked by HPLC.” The palladium(H) chelate was made according to an established procedure.‘* Dimethylformamide (DMF) solutions of PAN2 (lo-’ or 10-2M) were used for complexation, the optimal pH values being attained by addition of hydrochloric acid and sodium hydroxide solution, and monitored notentiometrically. The conditions for separating Pd(IIi Cu(II), NifII) and Co(II1) bv HPLC were chosen bv use of the PAN:2 chelates of thkse metals obtained under the conditions optimal for formation of the Pd(I1) chelate. The chelates was separated with a Varian-5000 liquid chromatograph equipped with a spectrophotometric detector (200-700 nm), and stainless-steel columns (250 x 5 mm) packed with Silasorb-600 silica gel (Lachema, CSSR) with an average particle size of 10 pm. Acetone, benzene, propan-2-01, chloroform and their mixtures of various
composition were used as eluents. All the organic solvents used were analytically pure. The capacity factors were determined in the usual way, with quinalizarin as the model non-adsorbed substance. The resolution (R,) for adjacent peaks was also calculated in the usual way.19 Calibration graph
Since the chromatographic peaks of the palladium chelate with PAN-2 have the form of Gauss curves, the peak height can be plotted against amount of palladium for calibration. Solutions containing 1.86-186 pg of palladium are each mixed with 5 ml of lo-“M PAN-2 solution in DMF and diluted with water to 25 ml after pH adjustment to 2.9-3.2. After mixing, the solutions are heated for l-2 min in a boiling water bath and then cooled. Next, 1 ml of chloroform is added to each, the mixtures are shaken for 1 min, and the extracts are separated. For chromatographic analysis, a lO-~1 aliquot part of the chloroform extract is injected by sample-loop into the HPLC column and chromatographed with acetone as eluent at a flow-rate of 2 ml/mm, at 25”. The absorbance of the ehtate at 620 nm is monitored. The calibration graph is linear over the palladium range 1.86186 pg in the initial solution.
Table 1. Optical characteristics of metal chelates with PAN-2% Central ion
1mu,-
Pd(I1) Ni(I1) Cu(I1) Co(II1)
620; 675 530; 565 550 580; 630
c, lo4 I.mole-‘.cm-’ @, am) 1.60 (620) 4.96 (565) 2.50 2.31 (580) 1.95 (630)
for the complexation reaction, the chromatographic detection of all the chelates was performed at 620 nm to avoid the background signal of PAN-2. At 620 nm the PAN-2 signal is not significant, and the palladium complex exhibits maximum absorption. To choose the best conditions for the separation, the nature and the composition of the mobile phase were varied, chloroform, acetone, benzene, propan2-01 and their mixtures being used as eluents. The palladium and nickel PAN-2 complexes are eluted with choroform and acetone to give sharp and symmetrical peaks and exhibit comparatively short retention times (Fig. 1). The retention volumes for these compounds do not depend on the size of the sample, which indicates that the chromatography of these chelates occurs in the linear region of their adsorption isotherms. Therefore, these components can be identified in a mixture by measuring the retention volumes. With acetone as eluent, the copper and cobalt chelates with PAN-2 are strongly retained, and their elution peaks, though symmetrical, are very broad. Although a mixture of the PAN-2 chelates of Pd(II), Cu(I1) and Co(II1) can be readily separated and palladium detected, the resolution is not good enough for complete separation of the palladium and nickel chelates. A weaker eluent, chloroform, also did not allow chromatographic separation of the four chelates, the cobalt chelate being so strongly retained that
Procedure
Pd (251
Five ml of O.lM PAN-2 solution in DMF are added to a known volume of sample solution containing 3-180 pg of palladium and not more than 2.5 mg of copper and 1.8 mg of cobalt. After adjustment of the pH to 2.9-3.2, the mixture is diluted to 25 ml with water, aud then heated, extracted etc., as in the procedure above for calibration.
RESULTS
AND DISXJSSION
The absorption spectra of the chelates studied have their absorption maxima in the visible region of the spectrum (Table l), where they differ from the PAN-2 spectrum (,l_ =470 mn). They coincide with the PAN-2 spectrum in the ultraviolet region. Because of the overlap of their major spectral bands, there is mutual interference between the palladium, cobalt, copper and nickel complexes, so a separation is needed. Because of the large excess of PAN needed
Co (321
cu (9.5) 1’-) 0
w I 10
I 20 t (min)
1 30
Fig. 1. Chromatogram of a mixture of the PAN-2 chelates of palladium, copper and cobalt. Eluent acetone, flowrate 2 ml/min, adsorbent IO-pm Silasorb-600, P = 28 atm, temperature 25”. The retention volumes (ml) corresponding to the peak maxima are. given in parentheses.
225
HPLC of PAN chelates Table 2. Retention parameters of PAN-2 chelates of palladium, nickel, copper and cobalt with the benxencpropan-2-01 1: 1 v/v mixture Retention time, ‘a 1 min 22 set 4 min 28 min 31 min 40 set
Central ion Ni(I1) W(H) Cu(II) Co(II1)
Capacity factor, K
Peak resolution, 4
Plate number, N
0 1.67 17.67 20.11
1 6.4 1.5
100 50 2x 10’ 2x 10’
Pd
it was not eluted even with a very large volume of eluent, and the palladium and nickel peaks were not
well separated, the resolution factor being only 0.6 (Fig. 2). Complete separation of all four components was attained with a 1: 1 v/v mixture of propan-2-01 and benzene (Fig. 3, Table 2). The copper and cobalt chelates have considerable retention times, and their separation requires greater consumption of eluent, and gradient elution with increasing polarity of the mobile phase is recommended for practical application. The column used in this work has been operated continuously for two months without change in the retention parameters or deterioration in the reproducibility of the peak heights. In the triplicate determination of 23.25 pg of palladium in the presence of 2.33 mg of copper and 1.86 mg of cobalt 23.1 kO.5 pg of palladium were found, showing that copper and cobalt do not interfere in the determination. The detection limit for palladium (with a lo-p1 injection) is 19 ng in the initial sample, the concenNi (1.26)
I
Pd (2.12)
0
10
20
30
t (mln)
Fig. 3. Separation of palladium, nickel, copper and cobalt PAN-2 chelates. Eluent 1: 1 v/v mixture of benzene and propan-2-01, P = 32 atm. Other conditions as for Fig. 1.
tration coefficient being 25. However, the detection limit can be decreased further by use of extractive chromatographic concentration. The method for palladium with PAN-2 solution in pentan-2-01 on polytetraiIuoroethylene (Teflon+ is characterized by a concentration coefficient of loO;*’ this should enable us to develop a new combination method involving the preliminary extractive chromatographic concentration of palladium from dilute solution, and its subsequent determination by the HPLC method.
REFERENCES
,J2_, , cu (9.3)
0
5
15
10
t
20
(mm)
Fig. 2. Chromatogram of a mixture of nickel, palladium and copper PAN-2 chelates. Eluent chloroform, P = 32 atm, 0th~ conditions as for Fig. 1.
1. A. R. Timerbaev, 0. M. Petrukhin and Yu. A. Zolotov, Zh. An&. Hum., 1981, 36, 1160. 2. J. W. O’Laughlin, J. Liquid khromatog., 1984,7, Suppl. 1, 127. 3. A. R. Timerbaev, 0. M. Petrukhin and Yu. A. Zolotov, Zh. Analit. Khim., 1982, 37, 581. 4. K.-H. Kbnig, G. Schneeweis and B. Steinbrech, Z. Anal. Own., 1983, 316, 13. 5. A. R. Timerbaev, 0. M. Petrukhin and Yu. A. Zolotov, Zh. Analit. Khim., 1982, 37, 1360. 6. A. R. Timerbaev and 0. M. Petrukhin, ibid., 1984, 39, 1177. 7. Idem, Anal. Chim. Acta, 1984, 159, 229. 8. E. Gaetani, C. F. Laureri, A. Mangla and G. Perolari, Anal. Chem., 1976, 4% 1725. 9. P. J. Clark, I. E. Treble and P. C. Uden, Polyhedron, 1982, 1, 785. 10. F. H. Walters, Anal. L&t., 1982, 15, 1031. 11. P. C. Uden, D. M. Parees and F. H. Walters, ibid, 1975, 8, 795.
226
Yu. S. NIKITIN ef 01.
12. V. M. Ivanov, Heterocyclic Nitrogen-containingArocompounds, (in Russian), p. 229. Nauka, Moscow, 1982. 13. S. V. Galushko, I. P. Shishkina and Yu. I. Usatenko, Zh. Analit. Khim., 1982, 37, 1833. 14. G. Schwedt and R. Budde, Chromatographia,1982, 15, 527. 15. F. Beamish, Analytical Chemistry of Noble Metals (in Russian). Part 2, p, 41. Mir, Moscow, 1969. 16. V. M. Ivanov, op. cit., p. 21.
17. S. N. Lanin, N. B. Morozova, V. M. Ivanov and E. M. Basova, Tezisy Dokl. Vses. Konf. po Analit. Khim. Organich. Soedineni, Moscow, 1984, 151. 18. V. M. Ivanov, op. cit., p. 55. 19. H. Engelhardt, High-Pressure Liquid Chromatography (in Russian), p. 20. Mir, Moscow, 1980. 20. V. M. Ivanov, op. cit., p. 51. 21. T. A. Bol’shova, N. B. Morozova and V. M. Ivanov, Tezisy Dokl. VII Vses. Konf. po Khim. Ekstrakt., Moscow, 1984, 147.
